I'm jumping the gun here, but something someone else posted reminded me...
We have Navigo cards that we got in Paris about 6 years ago. I kicked myself for not remembering them when we were in Paris again this year. My question is: Can we take our old Navigo cards next year, when we're going to be spending about a week or so in Paris? I know we'd have to load more money on them, but will they still be valid?